1. Field of the Invention
The present invention relates to a method for inspecting conditions under which semiconductor laser diodes (hereinafter called LDs) output a fixed wavelength and a unit for the same.
2. Description of the Related Art
FIG. 1 shows the construction of a general distributed feedback LD. In the LD, when a current flows into the PN junction portion in an active layer 11 from the upper electrode 12 to the lower electrode 12 and the recombination of electrons and holes takes place in the vicinity of the junction, the spontaneous emission of light corresponding to the band gap of the semiconductor in the active layer takes place. When the light having certain wavelength in accord with an interval of groove 13 travels in LD, optical amplification takes place and LD oscillates at the wavelength. It is well known that the change of the temperature of the device or heat generated due to injected current generally makes occurrence of expansion or contraction of the semiconductor crystal such as the active layer and the grooves of LD, and that the optical characteristics of thus oscillating LD depends on the temperature of the device or heat generated due to injected current as above described, so, as a result, the oscillation wavelength is shifted. Furthermore, the amount of the shift of the oscillation wavelength of each LD may be somewhat varied in accordance with the accuracy of the interval of the groove, the value of the drive current, and the crystal structure of the active layer. Therefore, it is required to inspect precisely the temperature dependency of the oscillation wavelength of each of the LDs.
FIG. 2 shows a wavelength inspection unit.
In the drawing, an LD module 1 is shown, and, inside the module 1, an LD, a laser mount for mounting the LD, a thermistor for monitoring the temperature of a heat sink portion to exhaust heat of the mount, and a Peltier element to cool and heat the laser mount (not shown) are mounted. A measurement jig 2 contains an electrical and thermal interface to the LD module 1. A laser driving power source 4, a temperature controller 5, and an photo detection portion 6 are put together in a current-light output measurement unit (hereinafter called an I-L measuring unit) 3. The photo detection portion 6 monitors the optical power of the LD through a fiber 9. A wavelength measurement apparatus 7 counts the output wavelength of the LD. A computer 8 gives control data to each device, receives data obtained at each device, and processes the data and controls the system in accordance with the measurement procedures.
FIG. 3 shows a conventional example of a wavelength tuning process for tuning an LD to a fixed wavelength in a case in which the wavelength inspection unit is used and a wavelength varying item is a laser temperature. In this case, since the laser temperature cannot be directly measured, the temperature of the heat sink on which a LD is mounted is defined as the laser temperature.
For example, the initial temperature of the heat sink portion of the LD is set at 25xc2x0 C. A current I, which is larger than a threshold current for laser oscillation, is applied to the LD, and a wavelength xcex0 is measured. The ratio of the change of oscillation wavelength to the change of heat sink temperature is defined as a wavelength coefficient, and its initial value is defined as a basic wavelength coefficient A0. Therefore, a heat sink temperature T1 is required to be (xcexTxe2x88x92xcex0)/A0+25xc2x0 C. in order to obtain a target wavelength xcexT. A wavelength xcex1 is measured at the temperature, and it is checked whether or not the difference between the wavelength xcex1 and the target wavelength xcexT is in an allowable range. If it is out of specification, it is checked whether the difference is in another fine adjustment window (the range of wavelength is slightly wider than the allowable range). If the difference is within the another fine adjustment window, the wavelength xcex1 can be broght to the target wavelength xcexT by changing the temperature a little. Then, a process for fine tuning, in which the temperature of the heat sink portion is changed by a certain temperature step and the measurement is continued until the oscillation wavelength reaches the target wavelength, is performed. On the other hand, if the difference is out of the fine adjustment window, the heat sink temperature T2 is set to (xcexTxcex1)/A0+T1 by using the target wavelength xcexT and the basic coefficient A0. Hereinafter, in the same way, an nth wavelength xcexn is measured, and then the wavelength xcexn is brought close to the target wavelength xcexT by continuing the operation until the difference comes within the allowable range or the fine adjustment window.
In the conventional method, it is possible to set the heat sink temperature for making an LD oscillated at a wavelength close to a target wavelength, but it is required to adjust the oscillation wavelength of the LD to the target wavelength in a further shorter time.
The initial basic wavelength coefficient used in the conventional method may be a theoretical value or an actual value, and it is a value which is characterized in that the amount of change of a wavelength is made to correspond to the amount of change of a wavelength varying item. However, the actual wavelength coefficient of each LD is different from each other. Therefore, when wavelength coefficients which have large differences from each other are used, the wavelength varying item is needed to be changed by large amount to result in a large difference in wavelength. Accordingly, a number of trial is required in order to reach the target wavelength.
FIG. 4 shows the difference between the conventional tuning method and a tuning method of the present invention. The solid line shows a characteristic of wavelength to temperature having a basic wavelength coefficient A0, and the dotted line shows the characteristic of wavelength to temperature of an LD to be tested. When the temperature of a heat sink portion is T0 and an oscillation wavelength is xcex0 at the beginning, the temperature T1 at the crosspoint of the straight line having the basic wavelength coefficient A0 and a straight line through a target wavelength xcexT is defined as a first setting temperature of the heat sink. An oscillation wavelength xcex1 at this crosspoint is shown by point B. In the conventional method, a straight line having the same inclination as the basic wavelength coefficient A0 is drawn from point B and the temperature is set to the temperature pointed at the crosspoint C1 of the straight line and the straight line through the target wavelength xcexT to obtain the point D1 for a second wavelength. On the other hand, in the present invention, an alternate long and short dash line having the inclination defined as the first wavelength coefficient A1 from point B is drawn, and the crosspoint C2 of the line and the straight line through the target wavelength xcexT is obtained. Since the wavelength when the temperature is set at this C2 is a wavelength pointed by D2, it is clearly understood that the wavelength pointed by D2 is closer to the target wavelength xcexT than the one pointed by D1.
In a wavelength inspection method of the present invention, tuning is performed such that a corrective wavelength coefficient in stead of the basic wavelength coefficient is obtained based on the amount of change of a wavelength varying item and the amount of change of a measured wavelength to the target wavelength, which is arisen from the above change of the wavelength varying item.
Furthermore, in a wavelength inspection method according to the present invention, a corrective wavelength coefficient is obtained in tuning of the first time and, after that, tuning is performed by using the so obtained corrective wavelength coefficient as a basic wavelength coefficient.
Furthermore, in a wavelength inspection method according to the present invention, tuning is performed each time by determining a corrective wavelength coefficient.
Furthermore, in a wavelength inspection method according to the present invention, the wavelength varying item is limited to the temperature or injected current of an LD. However, since the temperature of an LD cannot be directly measured, the temperature of the heat sink on which the LD is mounted is used as a substitute.
Furthermore, in a wavelength inspection method according to the present invention, the basic wavelength coefficient or the corrective wavelength coefficient when a measured wavelength reaches the target wavelength, is used as a basic wavelength coefficient when tuning of another LD is performed. Here, the basic wavelength coefficient is used only when the target wavelength is realized in tuning of the first time.
Furthermore, in a wavelength inspection method according to the present invention, when tuning of a plurality of LDs is performed, the average of the final wavelength coefficients (basic wavelength coefficients or corrective wavelength coefficients) already obtained from plural LDs is used as a basic wavelength coefficient in tuning the wavelength of a next one.
Furthermore, in a wavelength inspection method according to the present invention, when tuning of a plurality of LDs is performed, the final wavelength coefficient (basic wavelength coefficient or corrective wavelength coefficient) when tuning of a first LD has been performed is used as a basic wavelength coefficient of all the remaining LDs.
Furthermore, in a wavelength inspection method according to the present invention, when tuning of one LD is performed for a plurality of target wavelengths, tuning for each target wavelength is performed by determining a corrective wavelength coefficient each time.
Furthermore, in a wavelength inspection method according to the present invention, when tuning of one LD is performed for a plurality of target wavelengths, a corrective wavelength coefficient determined in tuning of the first time is used as a basic wavelength coefficient when tuning of the remaining target wavelengths is performed.
Furthermore, in a wavelength inspection method according to the present invention, when tuning is performed for a plurality of target wavelengths, the average of the final wavelength coefficients (basic wavelength coefficient or corrective wavelength coefficient) of some is used as a basic wavelength coefficient in tuning a next one.
Furthermore, in a wavelength inspection method according to the present invention, the average of the final wavelengths when tuning of one LD is performed for a plurality of target wavelengths is used as a basic wavelength coefficient in tuning another LD.
Furthermore, in a wavelength inspection method according to the present invention, when tuning for a plurality of target wavelengths is performed, only in tuning of the first time for a target wavelength, the final wavelength coefficient is obtained by determining a corrective wavelength coefficient, but, in tuning of the others, the final wavelength coefficient is used as a basic wavelength coefficient and the tuning is performed based on the conventional method.
A wavelength inspection unit according to the present invention is provided with a laser control portion for controlling a wavelength varying item to LDs, a wavelength measuring portion for measuring an oscillation wavelength, and a computer storing a basic wavelength coefficient showing a basic value of the ratio between the amount of change of the wavelength varying item and the amount of change of a wavelength caused thereby, performing an operation of a corrective wavelength coefficient based on the change of wavelength caused when the wavelength varying item is practically changed, performing an operation of controlling the wavelength varying item so that a wavelength reaches a target wavelength by using a basic wavelength coefficient or a corrective wavelength coefficient, and controlling tuning of a wavelength by giving and receiving data such as a wavelength, temperature, current, control signal, etc., between devices.
Furthermore, in a wavelength inspection unit according to the present invention, a laser control portion comprising a temperature control portion for controlling the temperature of a LD and a current control portion for supplying a current to the LD is contained.
Furthermore, in a wavelength inspection unit according to the present invention, a computation of the value of a wavelength varying item by which a wavelength can be brought to a target wavelength is performed by using a corrective wavelength coefficient, and the laser control portion controls a LD by using the value of a wavelength varying item.
Furthermore, in a wavelength inspection unit according to the present invention, the wavelength of a LD is measured at the value of a wavelength varying item determined by an operation portion and a corrective wavelength coefficient is corrected once again based on the amount of change of the wavelength varying item and the difference between a measured wavelength and a target wavelength.